Physical Characterization of Cathodically-Activated Corrosion Filaments on Magnesium Alloy AZ31B

In this study, the filiform-like corrosion behavior of Mg alloy AZ31B was characterized using analytical microanalysis. Recent electrochemical measurements have shown that the corrosion filaments on AZ31B support enhanced cathodic reaction kinetics, but there has been little strong physical evidence published to explain this behavior. Hence, the specific aim of the investigation was to contribute to the understanding of the physical origin of the “cathodic activation” of the corrosion filaments. Highlights of this investigation include the presence of through-thickness cracks within the corrosion filaments, Al-Mn particles (identified as Al₁₁Mn4 with energy-dispersive x-ray spectroscopy (EDS) quantification) in the preexisting films and filament corrosion products, and a significant enrichment of Zn at the corrosion filament/metal interface after aging of the filament. Diffraction patterns of the corrosion filaments indicated they were composed of nanocrystalline magnesium oxide (MgO), which was suggested as the product of the direct reaction of Mg and H₂O responsible for rapid H₂ evolution at the propagating corrosion front of the filaments.